Collecting Duct • 2 cell types o Principal cell o Intercalated   • α β Functions o Water reabsorption  urine concentration o H+ and K+ excretion o Bicarbonate excretion under specific circumstances • ADH (aka. Vasopressin) stimulates the insertion of aquaporins into luminal and basal membranes of CD cells Aquaporins • • • • Different isoforms of protein AQP1-4 AQP1 is found in the PCT AQP2 is phosphorylated and inserted into apical membrane of CD cells by ADH AQP3/4 produced in basolateral membrane of principal cells and not affected by ADH Ecstasy • • Stimulates ADH Life threatening dilution of body electrolytes  hyponatraemia  seizures Principle Cell Transport • • • Electrochemical gradient driven by Na+/K+ ATPase The cell now has a negative charge, excess K, low Na Na is absorbed into cell via ion channels, causing the lumen to become negatively charged • K can leave cell via apical and luminal channels but luminal excretion is favoured because the lumen’s negative charge attracts K • Expression of both channels is under the control of aldosterone (a steroid hormone that increases DNA transcription) α-Intercalated Cell Transport • Excrete H+ via o H+ pump (active transport) o H+/K+ exchanger (minor role) • Basolateral Cl-/HCO3- exchanger is used to reabsorb bicarbonate Net effect: H+ excretion • β-Intercalated Cell Transport • Mirror image of α-intercalated cell Excretes bicarbonate and reabsorbs H+ ions • Net effect is H+ reabsorption • For Countercurrent Multiplier and Exchanger, see Functional Structure of the Nephron Urea Recycling Summary: 1. Urea in medullary CD is concentrated because water is removed via upstream aquaporins 2. This produces a concentration gradient between luminal urea and adjacent interstitum 3. When ADH increases urea permeability in MCD, urea moves into the interstitum 4. There is now more urea in the interstitum than in the tALH 5. Urea enters the tALH and hence is recycled. This maintains medullary hypertonicity Urinary Dilution • • Necessary to conserve body solutes and to excrete excess water The early distal tubule (also called the cortical diluting segment) mostly dilutes the urine. This is because it’s downstream of the TAL and can still dilute the urine if TAL is malfunctioning • The TAL has a minor role in regulating dilution